Enzyme-linked immunosorbent assay (ELISA) is a sensitive technique for detecting and quantifying specific proteins (antigens or antibodies) in biological samples based on antigen-antibody interactions. There are four main ELISA types: sandwich, direct, indirect, and competitive, each with distinct protocols and applications. ELISA is widely used for quantitative and qualitative analysis of hormones, tumor markers, and antibodies in various biological fluids, offering high sensitivity and specificity.